Although one of our major medical problems is drug toxicity, we have practically no quantitative information on the relative effects that commonly used psychopharmacologic agents exert on the body's capacity to metabolize other drugs. This project was designed to provide such information. Plasma antipyrine half-lives will be measured before and at weekly intervals after the daily administration of the following drugs in usual therapeutic doses: phenobarbital, imipramine, amitriptyline, hydroxyzine, meprobamate, diazepam, thioridazine, promazine, trifluoperazine, chlorpromazine, and phenelzine. Knowledge of the relative potencies of these psychopharmacologic drugs as inducing agents will permit physicians to use them in combination with other therapeutic agents with less threat of toxic side effects. Also, such knowledge may provide a better insight into the chemical characteristics that confer inducing properties on a drug.